The invention is based on an exhaust-gas recirculating system for an internal-combustion engine.
As is known, the noxious emissions of an internal combustion engine can only be reduced to a certain point by optimization of the combustion chamber and of the fuel injection system. Hydrocarbon (HC) emissions are minimized by advancing the beginning of injection, which increases NO.sub.x emissions. For the reduction of NO.sub.x emissions, it is well known to employ exhaust-gas recirculation (EGR), for which various methods have been developed. This advancing of the beginning of injection, effected specifically to minimize HC emissions, is partly overlapped by the load and speed-dependent regulation of the beginning of injection, for which provision has already been made in the fuel-injection system. As is known, the beginning of injection has to be advanced as the speed increases, particularly in order to compensate for the natural retardation of the injection with increasing speed, which is due to the angularly increasing propagation time of the pressure waves in the fuel injection line. Exhaust-gas recirculation must only be effected below a certain load--that is, below a certain injection quantity--which changes with the speed approximately according to the characteristic load curves of the governor.
A prior art exhaust-gas recirculating system for internal-combustion engines of this type is known from the published German patent application OS 29 46 557.4. In this system a distributor-type pump is used as a fuel injection pump in which delivery is initiated by means of a speed-dependent hydraulic pressure prevailing in the suction chamber of the pump and wherein that pressure is influenced by a discharge opening that is dependent on the position of the sleeve of the centrifugal governor. The position of the governor sleeve is a function of speed and load since the governor sleeve is subject to both the strictly speed-dependent centrifugal forces and, in the opposite direction, the forces which correspond to the load, applied through a governor spring and variable at will by means of the control lever. The discharge port is uncovered by the governor sleeve when the load and speed desired for exhaust gas recirculation are reached. The fuel draining through the discharge port is constricted by means of a throttle, with the back pressure then acting on a pressure-sensitive switch in an electric line of the solenoid of the directional control valve. Moreover, a switch inserted in that line is actuated by a trip cam on the control lever for a specific load range. When both switches are closed, the directional valve shifts so that the pneumatically actuated exhaust-gas recirculation valve is opened. The system thus provides an on/off control of exhaust-gas recirculation rather than a continuous control.
Since it uses a hydraulic pressure switch and a camoperated switch, this type of control is very costly, and also disadvantageous because of the space it requires, as well as being prone to malfunction, quite apart from the fact that the constricting throttle has a specific cross-sectional area which, from a given volume of outflow on, produces the necessary switching pressure. However, with larger volumes being discharged per unit time--that is, with high speeds and with the discharge port opened--this results in the throttle acting as a closure so that the variation of the pressure gradient in the intake chamber intended for adjustment of the beginning of injection does not produce the desired result.